5.1 beam by electron bombardment of C₆₀ vapor

R. Vandenbosch and D.I. Will

Efforts to produce C₆₀ anions directly from graphite in our General Ionex model 860 sputter ion source have proved fruitless. We have, however, seen nanoamp currents of sputtered from C₆₀ substrate in this source. These currents always decayed in less than an hour to unusable levels. As a result we undertook to develop a new ion source. One of our colleagues, Adi Scheideman, was producing a neutral C₆₀ beam from C₆₀ powder placed in a quartz oven at 300 to 600° C. Reports that neutral C₆₀ has a sizable capture probability for electrons impinging at energies from 2 to 20 eV induced us to try ionizing such a molecular beam by electron bombardment.

The geometry of our new ion source is largely cylindrically symmetric about the C₆₀ beam axis. A quartz oven emits C₆₀ vapor from a 1 mm orifice into a drift tube where the vapor is ionized by electron capture from an electron beam crossing the C₆₀ beam. The resulting is accelerated forward by a potential of +200 V on the first electrode at the end of the drift tube. This beam is then focused by a second electrode at roughly +900 V (varied for best focus). Final acceleration is to a total energy of 6 kV (limited by the capability of our 90° analyzing magnet).

Some aspects of our design are unique to the source (though the focus and final acceleration electrodes are unchanged from our direct extraction ion source). The quartz oven of 7 mm outside diameter by 50 mm length is heated by a nichrome coil wrapped tightly on the oven. An insulating blanket of alumina fiber felt is wrapped over the nichrome. A Pt-Pt13%Rh thermocouple monitors the oven temperature, and the oven heater power supply is then driven by a regulator circuit to maintain whatever constant temperature is preset. The C₆₀ vapor leaves this oven through the 1 mm orifice (at one end of the oven) and immediately enters the drift tube made of 304 stainless steel which is 12 mm inside diameter by 50 mm long. A coiled tungsten filament is located in a hole in the side of the drift tube roughly 8 mm downstream of the oven. The potential of this filament is variable from 0 to -20 volts relative to the drift tube. In operation the filament is heated by roughly 6 amps DC from a voltage regulated supply and emits 1 to 2 mA of electrons directed crudely into the interior of the drift tube.

The beam, once analyzed by the 90 degree magnet, is collected in a removable Faraday cup or can be accelerated off our injector deck by an additional 300 keV. To date we have observed up to 10 nA of analyzed beam into our on-deck Faraday cup with oven temperatures around 650 degrees C. Of more interest, we can achieve stable beams of several hundred pA and maintain them for more than a day at oven temperatures around 500° C. On disassembly after a day's run we find most of the roughly 100 mg oven load of C₆₀ deposited on the first electrode at the end of the drift tube. Based on rough integration of the beam seen on the Faraday cup over a day's time relative to the C₆₀ load vaporized from the oven, our ionization efficiency is low, perhaps 1 part in 10⁶. We find peak C₆₀ electron capture for energies of 5 to 6 eV, in rough agreement with reported values.